Reduction of nitrates



Patented Sept. 1, 1942 REDUCTION OF NITRATES John R. Bates, Swarthmore, Pa., assignor to Houdry Process Corporation, Wilmington, Del., a corporation of Delaware No Drawing. Application September 27, 1940, Serial No. 358,706

3 Claims.

The present invention relates to the reduction of alkali metal nitrates and has special reference to the controlled reduction of such nitrates for the production therefrom of nitrites of the alkali metal or metals present.

Previously the controlled reduction of alkali metal nitrates has been efiected by contacting the molten salt with molten lead. Such a procedure is desirable in that virtually none of the reduction proceeds beyond the nitrite stage. However, it is substantially disadvantageous due to the salt becoming contaminated with lead oxide, which presents a purification problem.

In accordance with the present invention a molten salt composition containing alkali metal nitrate is contacted at an elevated temperature with hydrogen, whereby reduction to nitrite is effected and salt so reduced is contacted at a lower temperature with unreacted gas from the zone in which the reduction is efiected.

While reference is made herein to the treatment of molten salts, it should be understood that by the term molten herein and in the subjoined claims it is intended to indicate liquid compositions including those which contain suspended solids. r

The molten salt composition treated in accordance with this invention may comprise principally alkali metal nitrate or may contain alkali metal nitrate, in quantity preferably above about 30 5% of the total composition, together with diluent materials such as free alkali, alkali metal carbonate, nitrite or halide. Thus, a special application of this invention is in the regeneration of alkali metal nitrate-nitrite salt compositions 35 used as heat transfer media. Such a salt may, for example, contain initially 54% potassium nitrate and 46% sodium nitrite. After extended use the composition of the salt changes gradually until a major portion or substantially all of the 40 nitrite is converted to nitrate. This invention is applicable to the treatment of such salts, either the initial salt in order to increase the nitrite or after a period of use in the molten state to reduce the melting point.

In the higher portion of the temperature range between the melting point of the composition containing alkali metal nitrate and the temperature at which substantial decomposition of the nitrate occurs, hydrogen, when contacted with 50 the salt, effects reduction of the nitrate to nitrite. In the case of compositions of sodium and potassium nitrate and nitrite the safe upper temperathe order of 750 F. or slightly thereabove. During this reduction, a small percentage 01 free alkali is formed, which may be due to a small proportion of the reduced material going beyond the nitrite stage. Continued contact of the salt with unreacted gas at the high temperature eifects further reduction of nitrate accompanied by further production of alkali.

After the reduction has been carried as far as desired the reduced salt is contacted with gas from the reduction zone, the temperature of the contact being between the melting point of the reduced salt and the lowest temperature at which substantial reduction with hydrogen may be effected, preferably between about 500 and 750 F. Contact of this gas, which is predominantly hydrogen, with the salt composition at this lower temperature effects a substantial reduction in the alkali content.

The temperature of the composition may be reduced between the two treating zones by passing it through a heat exchanger to control the temperature as desired. Alternatively, a tall tower may be employed for the treatment, and both the salt composition and the hydrogen may be fed to the lower end of' the tower. As the composition flows upwardly in the tower the temperature gradually drops, whereby the lower portion of the tower acts as a reducing zone and the upper portion of the tower acts as a zone for the treatment of the composition with gas from the reduction zone. Gas from the zone in which the alkali content is reduced may be returned for the treatment of fresh salt, preferably after removal of water vapor by condensation.

Illustrative of this invention, a salt composition which had been used as a heat transfer ture is about 1000 to 1100 F. The temperature above which substantial reduction occurs is of medium at a temperature of about 850 F. which originally contained about 54% potassium nitrate and 46% sodium nitrite and which after use contained about 51% potassium nitrate, 22% sodium nitrate, 26% sodium nitrite and about 1% alkali (including free alkali and alkali carbonate, being calculated as NaOH), was run to a reducing column wherein it was contacted with hydrogen at a temperature of about 850 F. 10% of the hydrogen fed to the tower reacted per pass. The nitrate was reduced in 3 hours from 47.5% nitrate ion to 29.5% nitrate, which was accompanied with the production of about 0.75% free alkali. The temperature of the composition was reduced to about 650 F. and charged to a tower through which unreacted gas from the reducing tower were passed. Contact of the composition with the unreacted gas effected a substantial 2 reduction or the alk li present. This treatment eilected'a reduction in the melting point of the composition from about 330 F. to about 295 F.

While special reference has been made to the regeneration of heat transfer salts, it should be vunderstood that the present process is applicable to the commercial production of nitrites and therefore that the'scope of this invention should not be construedas limited by the disclosure but only by the appended claims. I

I claim as my invention:

1. The process for the reduction of alkali metal nitrate, comprising contacting in a reduction zone a molten composition containing alkali metal nitrate with hydrogen, maintaining the temperature of contact in the reduction zone at above about 750 F. whereby alkali metal nitrate and about 750 F.

tion containing alkali metal nitrate in a reduc ing zone with hydrogen. the temperature of contact in the reducing zone being above the lowest temperature at which substantial reduction of molten nitrate by hydrogen occurs and below the temperature at which substantial thermal decomposition of the nitrate occurs, and contacting molten composition from the reducing zone with gas from said zone at a temperature above the melting point of the composition and below the temperature at which substantial reduction with hydrogen occurs.

3. The process or treating a heat transfer medium which contains principally alkali metal nitrate and nitrite to efiect decrease oi the percent nitrate and increase in the percent nitrite which process comprises contacting the medium in molten state at a temperature above about 750 F. with a gas containing hydrogen, and contacting molten medium from the first contacting step with gas from said first step at a temperature between the melting point of the medium JOHN- R. BATES. 

